1. Field of the Invention
The present invention relates generally to water screens comprising a revolving screen member, e.g., a band screen or drum screen, commonly used in industrial and agricultural water intakes.
2. Description of the Prior Art
Water intakes are usually situated on the shore of a river, lake or ocean. The water taken in inevitably contains varying quantities of foreign bodies, comprising debris and bodies of natural origin such as leaves, grass, tree branches, fish, algae and seashells, for example, and man-made waste materials, including various containers and other forms of solid industrial waste.
It goes without saying that prior to use the water drawn in through such an intake must have these foreign bodies removed, to eliminate the risk of disrupting the operation of the installations supplied by the intake and shortening the service life thereof.
The removal of foreign bodies is usually carried out in two or more stages.
Initially, a bar screen composed of bars spaced apart a few millimeters or a few centimeters from one another arrests the largest foreign bodies.
In a second stage, on the downstream side of such a bar screen, a screen with a revolving screen member are used. The revolving screen may be a drum screen, for example, in which the screen member comprises cylindrical side wall of a rotating drum.
The present invention is more particularly though not exclusively, concerned, however, with traveling band screens, that is to say with screens comprising an endless screen band passing over a pair of spaced apart direction changing means for changing the direction of the path of movement of the screen band, at least one of the direction changing means being in driving relation with the screen band.
In practice, the screen band of this type of traveling band screen is formed by a succession of screening panels generally supported at respective sides by two chains which pass over a pair of direction changing means, e.g. sprockets, so that the resulting assembly constitutes an endless band passing around these direction changing means. As an alternative, the screening panels may be articulated to one another so as to form of themselves an endless band.
In use, the traveling band screen is disposed in a vertical or inclined position in a sluice forming part of the water intake it equips.
As a general rule, the traveling band screen extends generally transversely relative to the direction of flow of water in the sluice, the generatrices of its screening band being substantially perpendicular to this direction.
In such a case only the upstream run or face of the screening band is in contact with water to be severed, which passes through this upstream run or face and then through its downstream run or face of the screen band.
Such traveling band screens are known as flow-through traveling band screens, and offer the advantage of requiring a minimal installation volume, need only simple and therefore relatively inexpensive masonry structures, and produce no significant turbulence in the installations on the downstream side.
They do have disadvantages, however, of which of present concern is the fact that the outside surface of the screen band is alternately in contact with the water to be filtered, on its upstream side, and water already filtered, on its downstream side.
The foreign bodies to be eliminated accumulate on its upstream run or face and, once deposited on same by the water to be filtered, may be subsequently thrown off into the water already filtered, on the downstream side of the screen band, if the foreign bodies on the screen are not in the meantime removed.
It is thus imperative to provide, in the upper part of the filter, above the water, a washing device.
This normally comprises a sprayer disposed facing one side of one run of the screen band to be washed, to direct a jet of fluid (in practice a flat sheet) towards the screen band, and a recovery device, i.e., a recovery channel, conveyer, endless belt or other type, disposed on the opposite side of one run of the screen band to recover material detached therefrom and the sprayed fluid.
In practice, there is inevitably some gap or clearance between the recovery device and the screen band, to allow for the swinging movement of the latter when in operation. Because of this clearance, the washing device has previously been disposed preferably on the upstream run of the screen band.
If diposed on the downstream run or face of the screen band, in what is normally known as the "carry over" configuration, a significant fraction of the material detached from the screen band can contaminate the water already screened, by falling between the screen band and the recovery device, by virtue of the clearance or gap existing between the screen band and the recovery device.
When the washing device is disposed on the upstream run or face of the screen band material falling off the screen band has as its only consequence the increased cost of recycling this material, without polluting the water already filtered, since the material falls back into the water already filtered, since the material falls back into the water to be filtered.
Thus, for reasons of efficiency, it would be preferable for the washing device to be placed on the downstream run or face screen band.
The material deposited on the screen band builds up a layer of impurities which are detached in a single large piece or smaller parts when subjected to the action of the jet of sprayed-fluid from the sprayer. This detachment is progressive, however, beginning at the generatrix of the screen band where the sprayed fluid impinges the band, relative to the direction of movement of the screen band, this generatrix is a leading edge of the layer of impurities to be detached.
If the washing device is disposed on the upstream run or face of the screen band, so that the fluid spray acts on upward moving screening panels, impurities detached from the screen band as soon as they reach the lower generatrix or limit of the fluid spray, immediately fall under their own weight and thus are not subjected to the full horizontal propulsive force exerted by the fluid spray which is intended to entrain them into the associated recovery device.
On the other hand, if the washing device is disposed on the downstream run or face of the screen band, so that the washing spray acts on downward moving screening panels, the detached impurities fall into the fluid spray and are subjected to the full horizontal propulsive force exerted by the latter, which entrains them into the associated recovery device, although the heaviest particles may escape from the fluid spray and drop into the water already screened.
A general object of the present invention is to provide an arrangement whereby the washing device may be disposed on the downstream face or run of the screen band without any risk of contamination of the already screened water by materials detached from the screen band and escaping from the fluid spray.